The charge of rioting, which carries a maximum 10-year prison sentence, has not been used in Hong Kong in more than 15 years.

The BBC's Juliana Liu in Hong Kong says its use reflects the seriousness with which authorities are responding to the clashes.

The South China Morning Post said the last use of the rioting charge was after violent clashes between inmates at a drug treatment centre in 2000.

Mong Kok clashes

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Media captionThe BBC spoke to street hawkers in Mong Kok

What happened in Mong Kok?

Hong Kong police attempted to shut down unlicensed food stalls along the junction of Shan Tung Road and Portland Street in Kowloon. Clashes erupted after dozens of local activists gathered to defend the vendors.

Why now?

The unlicensed food stalls were set up for the Lunar New Year holiday. Officials usually turn a blind eye to the hawkers' lack of official permits, but this year decided to crack down.

Were the clashes just about street food?

The underlying tensions go deeper. Many "localist" groups, who want greater autonomy for Hong Kong, turned up to support the vendors, arguing that Hong Kong's identity is under threat. Trust between the public and the police has also declined in recent years. On social media the protest was dubbed #fishballrevolution, after one of the snacks on sale.